A color picture tube includes an electron gun for generating and directing three electron beams to the screen of the tube. The screen is located on the inner surface of a faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. A color selection electrode, which may be either a shadow mask or a focus mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is usually contoured to somewhat parallel the inner surface of the tube faceplate.
One type of color picture tube has a tension mask affixed to two parallel support frame members under tension and mounted within a faceplate panel thereof. The tension assists in controlling vibration, or microphonics, of the mask during tube operation. During the manufacturing of the CRT, the mask and frame are passed through a furnace and heated to very high temperatures thereby causing the frame and mask to expand. During this heating cycle, the mask and frame materials lose strength and deform beyond their plasticity limits. When the mask and frame are cooled thereafter the tension on the mask is reduced by approximately the ratio of the total plastic deformation to the original compliance. Conventional frame designs have high center compliance but very low edge compliance. The low edge compliance provides little reserve for tension control at the edges of the mask. The compliance pattern of the conventional frame results in mask deformation which can wrinkle the mask. In order to maintain the tension on the mask in a conventional tension mask frame, the mask must be attached to a relatively massive support frame. It has been suggested that a lighter frame could be used in a tension mask tube if the required tension on a mask is reduced. One way to reduce the required mask tension is to make the mask from a material having a low coefficient of thermal expansion. However, a mask from such material would require a support frame of a material having a similar coefficient of thermal expansion, to prevent any mismatch of expansions during thermal processing that is required for tube manufacturing. Because the metal materials that have low coefficients of thermal expansion are relatively expensive, it is relatively costly to make both the mask and frame out of identical or similar low expansion materials. Therefore, it is desirable to use the combination of a low expansion tension mask with a higher expansion support frame, and to provide a solution to the problem that exists when there is a substantial mismatch in coefficients of thermal expansion between a tension mask and its support frame. It is also desirable to provide a means for modifying the pattern of compliance of the mask frame to control tension on the mask lost through thermal processing and to prevent wrinkling of the mask.